The present invention relates to an improved needle and a method of making the same. More specifically, the present invention relates to needle electrodes of the type used in electromyography.
To facilitate an understanding of the present invention, it is helpful to first consider the terminology as used in this application with respect to needles in general and with respect to needle electrodes. Both needles and needle electrodes are usually formed with an elongated body which may be of circular or oval cross-section. The elongated body is often referred to as the shank. One end of the needle is sharpened, for insertion into the skin, and the leading portion of the needle, i.e. the first part of the needle to penetrate the skin, is referred to as the point. Typically, the shank is tapered (i.e., formed with an angularly disposed face) toward the point and the tapered surface of the shank is referred to as a bevel or facet. The point and bevel of the needle together are collectively referred to as the tip.
Heretofore, the bevel has been formed by grinding a flat plane which intersects the longitudinal axis of the shank at a desired angle. The intersection of the flat plane and the circular shank defines an edge, which, in plan view, is of elliptical configuration.
Hypodermic needles, which are a form of cannulas, have a hollow shank and the formation of the bevel as described actually forms two ellipses (in plan view); the first ellipse at the intersection of the bevel with the exterior surface of the shank and the second ellipse at the intersection of the bevel with the periphery of the hollow interior of the shank. The second ellipse has a greater cross-sectional area than the area of the hollow portion of the shank measured at right angles to the longitudinal axis of the shank and this greater cross-sectional area permits greater fluid flow through the needle thus aiding in dispersion of medications (or in withdrawal of fluids in the case of a cannula).
Needle electrodes, such as the type used in electromyography, typically include a shank which is formed of at least two elongated electrically conductive members or electrodes; the first or outer electrode surrounding the second or inner electrode. The inner and outer electrodes may be coaxially aligned and radially spaced apart with an annular cylinder of insulating material therebetween. If a third conductive electrode is present, the third electrode is usually positioned interiorly of the first or outer electrode and each of the electrodes is electrically insulated from the other electrodes. Needle electrodes as used in electromyography may be monopolar, bipolar, etc., depending upon the number of electrodes and the manner in which the electrodes are connected.
When the bevel of the needle or needle electrode is formed by cutting or grinding the shank at an angle to its longitudinal axis to form the point and the tip, neither the point nor the tip are laterally supported against forces exerted thereagainst during insertion of the needle or needle electrode. The lack of lateral support makes the tip vulnerable to breakage during insertion.
In electromyography, a needle electrode is inserted into a muscle and rotated in four 90.degree. increments. After each 90.degree. rotation, an electrical potential or response is measured. One needle electrode used in electromyography is the model 13L65 manufactured by Disa Electronics Division of Disamatic Inc. The model 13L65 electrode includes a bevel ground at a 15.degree. angle relative to the longitudinal axis of the shank. According to the manufacture, the 13L65 needle electrode should be reground after each 10-20 usages. After regrinding the bevel, the manufacturer recommends smoothing or removing the point with a grinding paper and this removes any burrs. However, this smoothing also blunts the point which adds to patient discomfort when the needle is inserted into the patient.
In addition, when a needle electrode is inserted into a patient and thereafter rotated, the edge (i.e. the intersection of the bevel and the shank) is of a rapidly increasing width in a direction away from the point and the edge does not easily cut tissue because the bevel is substantially flat. The tissue, rather than being easily cut by the rotating edge, is instead torn or stretched resulting in pain being incurred by the patient.